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    <h1>Vue 实例</h1>
  

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    <a class="dashAnchor" name="//dash_ref_创建一个-Vue-实例/Section/%E5%88%9B%E5%BB%BA%E4%B8%80%E4%B8%AA-Vue-%E5%AE%9E%E4%BE%8B/0"></a><h2 id="创建一个-Vue-实例">创建一个 Vue 实例</h2><div class="vueschool"><a href="https://learning.dcloud.io/#/?vid=2" target="_blank" rel="sponsored noopener" title="Vue.js 教程 - 创建第一个 Vue 应用">观看本节视频讲解</a></div>

<p>每个 Vue 应用都是通过用 <code>Vue</code> 函数创建一个新的 <strong>Vue 实例</strong>开始的：</p>
<pre><code class="hljs js"><span class="hljs-keyword">var</span> vm = <span class="hljs-keyword">new</span> Vue(&#123;
  <span class="hljs-comment">// 选项</span>
&#125;)</code></pre>
<p>虽然没有完全遵循 <a href="https://zh.wikipedia.org/wiki/MVVM" target="_blank" rel="noopener">MVVM 模型</a>，但是 Vue 的设计也受到了它的启发。因此在文档中经常会使用 <code>vm</code> (ViewModel 的缩写) 这个变量名表示 Vue 实例。</p>
<p>当创建一个 Vue 实例时，你可以传入一个<strong>选项对象</strong>。这篇教程主要描述的就是如何使用这些选项来创建你想要的行为。作为参考，你也可以在 <a href="https://cn.vuejs.org/v2/api/#选项-数据">API 文档</a> 中浏览完整的选项列表。</p>
<p>一个 Vue 应用由一个通过 <code>new Vue</code> 创建的<strong>根 Vue 实例</strong>，以及可选的嵌套的、可复用的组件树组成。举个例子，一个 todo 应用的组件树可以是这样的：</p>
<pre><code class="hljs undefined">根实例
└─ TodoList
   ├─ TodoItem
   │  ├─ DeleteTodoButton
   │  └─ EditTodoButton
   └─ TodoListFooter
      ├─ ClearTodosButton
      └─ TodoListStatistics</code></pre>
<p>我们会在稍后的<a href="components.html">组件系统</a>章节具体展开。不过现在，你只需要明白所有的 Vue 组件都是 Vue 实例，并且接受相同的选项对象 (一些根实例特有的选项除外)。</p>
<a class="dashAnchor" name="//dash_ref_数据与方法/Section/%E6%95%B0%E6%8D%AE%E4%B8%8E%E6%96%B9%E6%B3%95/0"></a><h2 id="数据与方法">数据与方法</h2><div class="vueschool"><a href="https://learning.dcloud.io/#/?vid=3" target="_blank" rel="sponsored noopener" title="Vue.js 教程 - 数据与方法">观看本节视频讲解</a></div>

<p>当一个 Vue 实例被创建时，它将 <code>data</code> 对象中的所有的属性加入到 Vue 的<strong>响应式系统</strong>中。当这些属性的值发生改变时，视图将会产生“响应”，即匹配更新为新的值。</p>
<pre><code class="hljs js"><span class="hljs-comment">// 我们的数据对象</span>
<span class="hljs-keyword">var</span> data = &#123; <span class="hljs-attr">a</span>: <span class="hljs-number">1</span> &#125;

<span class="hljs-comment">// 该对象被加入到一个 Vue 实例中</span>
<span class="hljs-keyword">var</span> vm = <span class="hljs-keyword">new</span> Vue(&#123;
  <span class="hljs-attr">data</span>: data
&#125;)

<span class="hljs-comment">// 获得这个实例上的属性</span>
<span class="hljs-comment">// 返回源数据中对应的字段</span>
vm.a == data.a <span class="hljs-comment">// =&gt; true</span>

<span class="hljs-comment">// 设置属性也会影响到原始数据</span>
vm.a = <span class="hljs-number">2</span>
data.a <span class="hljs-comment">// =&gt; 2</span>

<span class="hljs-comment">// ……反之亦然</span>
data.a = <span class="hljs-number">3</span>
vm.a <span class="hljs-comment">// =&gt; 3</span></code></pre>
<p>当这些数据改变时，视图会进行重渲染。值得注意的是只有当实例被创建时就已经存在于 <code>data</code> 中的属性才是<strong>响应式</strong>的。也就是说如果你添加一个新的属性，比如：</p>
<pre><code class="hljs js">vm.b = <span class="hljs-string">'hi'</span></code></pre>
<p>那么对 <code>b</code> 的改动将不会触发任何视图的更新。如果你知道你会在晚些时候需要一个属性，但是一开始它为空或不存在，那么你仅需要设置一些初始值。比如：</p>
<pre><code class="hljs js">data: &#123;
  <span class="hljs-attr">newTodoText</span>: <span class="hljs-string">''</span>,
  <span class="hljs-attr">visitCount</span>: <span class="hljs-number">0</span>,
  <span class="hljs-attr">hideCompletedTodos</span>: <span class="hljs-literal">false</span>,
  <span class="hljs-attr">todos</span>: [],
  <span class="hljs-attr">error</span>: <span class="hljs-literal">null</span>
&#125;</code></pre>
<p>这里唯一的例外是使用 <code>Object.freeze()</code>，这会阻止修改现有的属性，也意味着响应系统无法再<em>追踪</em>变化。</p>
<pre><code class="hljs js"><span class="hljs-keyword">var</span> obj = &#123;
  <span class="hljs-attr">foo</span>: <span class="hljs-string">'bar'</span>
&#125;

<span class="hljs-built_in">Object</span>.freeze(obj)

<span class="hljs-keyword">new</span> Vue(&#123;
  <span class="hljs-attr">el</span>: <span class="hljs-string">'#app'</span>,
  <span class="hljs-attr">data</span>: obj
&#125;)</code></pre>
<pre><code class="hljs html"><span class="hljs-tag">&lt;<span class="hljs-name">div</span> <span class="hljs-attr">id</span>=<span class="hljs-string">"app"</span>&gt;</span>
  <span class="hljs-tag">&lt;<span class="hljs-name">p</span>&gt;</span>&#123;&#123; foo &#125;&#125;<span class="hljs-tag">&lt;/<span class="hljs-name">p</span>&gt;</span>
  <span class="hljs-comment">&lt;!-- 这里的 `foo` 不会更新！ --&gt;</span>
  <span class="hljs-tag">&lt;<span class="hljs-name">button</span> <span class="hljs-attr">v-on:click</span>=<span class="hljs-string">"foo = 'baz'"</span>&gt;</span>Change it<span class="hljs-tag">&lt;/<span class="hljs-name">button</span>&gt;</span>
<span class="hljs-tag">&lt;/<span class="hljs-name">div</span>&gt;</span></code></pre>
<p>除了数据属性，Vue 实例还暴露了一些有用的实例属性与方法。它们都有前缀 <code>$</code>，以便与用户定义的属性区分开来。例如：</p>
<pre><code class="hljs js"><span class="hljs-keyword">var</span> data = &#123; <span class="hljs-attr">a</span>: <span class="hljs-number">1</span> &#125;
<span class="hljs-keyword">var</span> vm = <span class="hljs-keyword">new</span> Vue(&#123;
  <span class="hljs-attr">el</span>: <span class="hljs-string">'#example'</span>,
  <span class="hljs-attr">data</span>: data
&#125;)

vm.$data === data <span class="hljs-comment">// =&gt; true</span>
vm.$el === <span class="hljs-built_in">document</span>.getElementById(<span class="hljs-string">'example'</span>) <span class="hljs-comment">// =&gt; true</span>

<span class="hljs-comment">// $watch 是一个实例方法</span>
vm.$watch(<span class="hljs-string">'a'</span>, <span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params">newValue, oldValue</span>) </span>&#123;
  <span class="hljs-comment">// 这个回调将在 `vm.a` 改变后调用</span>
&#125;)</code></pre>
<p>以后你可以在 <a href="https://cn.vuejs.org/v2/api/#实例属性">API 参考</a>中查阅到完整的实例属性和方法的列表。</p>
<a class="dashAnchor" name="//dash_ref_实例生命周期钩子/Section/%E5%AE%9E%E4%BE%8B%E7%94%9F%E5%91%BD%E5%91%A8%E6%9C%9F%E9%92%A9%E5%AD%90/0"></a><h2 id="实例生命周期钩子">实例生命周期钩子</h2><div class="vueschool"><a href="https://learning.dcloud.io/#/?vid=4" target="_blank" rel="sponsored noopener" title="Vue.js 教程 - 生命周期">观看本节视频讲解</a></div>

<p>每个 Vue 实例在被创建时都要经过一系列的初始化过程——例如，需要设置数据监听、编译模板、将实例挂载到 DOM 并在数据变化时更新 DOM 等。同时在这个过程中也会运行一些叫做<strong>生命周期钩子</strong>的函数，这给了用户在不同阶段添加自己的代码的机会。</p>
<p>比如 <a href="https://cn.vuejs.org/v2/api/#created"><code>created</code></a> 钩子可以用来在一个实例被创建之后执行代码：</p>
<pre><code class="hljs js"><span class="hljs-keyword">new</span> Vue(&#123;
  <span class="hljs-attr">data</span>: &#123;
    <span class="hljs-attr">a</span>: <span class="hljs-number">1</span>
  &#125;,
  <span class="hljs-attr">created</span>: <span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params"></span>) </span>&#123;
    <span class="hljs-comment">// `this` 指向 vm 实例</span>
    <span class="hljs-built_in">console</span>.log(<span class="hljs-string">'a is: '</span> + <span class="hljs-keyword">this</span>.a)
  &#125;
&#125;)
<span class="hljs-comment">// =&gt; "a is: 1"</span></code></pre>
<p>也有一些其它的钩子，在实例生命周期的不同阶段被调用，如 <a href="https://cn.vuejs.org/v2/api/#mounted"><code>mounted</code></a>、<a href="https://cn.vuejs.org/v2/api/#updated"><code>updated</code></a> 和 <a href="https://cn.vuejs.org/v2/api/#destroyed"><code>destroyed</code></a>。生命周期钩子的 <code>this</code> 上下文指向调用它的 Vue 实例。</p>
<p class="tip">不要在选项属性或回调上使用<a href="https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Functions/Arrow_functions" target="_blank" rel="noopener">箭头函数</a>，比如 <code>created: () =&gt; console.log(this.a)</code> 或 <code>vm.$watch(&#39;a&#39;, newValue =&gt; this.myMethod())</code>。因为箭头函数并没有 <code>this</code>，<code>this</code> 会作为变量一直向上级词法作用域查找，直至找到为止，经常导致 <code>Uncaught TypeError: Cannot read property of undefined</code> 或 <code>Uncaught TypeError: this.myMethod is not a function</code> 之类的错误。</p>

<a class="dashAnchor" name="//dash_ref_生命周期图示/Section/%E7%94%9F%E5%91%BD%E5%91%A8%E6%9C%9F%E5%9B%BE%E7%A4%BA/0"></a><h2 id="生命周期图示">生命周期图示</h2><p>下图展示了实例的生命周期。你不需要立马弄明白所有的东西，不过随着你的不断学习和使用，它的参考价值会越来越高。</p>
<p><img src="../../images/lifecycle.png" alt="Vue 实例生命周期"></p>

  
  
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